1. Field of the Invention
The present invention relates to a recording head, and more specifically, it relates to an ink-jet recording head that discharges ink with a thermo-mechanical actuator to perform recording.
2. Description of the Related Art
Recording heads of ink-jet recording apparatuses use various methods for discharging ink. For example, a method in which thermal energy is applied on ink to generate a bubble and a method using a piezo element as an electro-mechanical actuator have been known and put to practical use. In addition, in recent years, the development of a method using a thermo-mechanical actuator has been advanced because the method affords greater manufacturability and flexibility in ink composition.
U.S. Pat. Appl. Pub. No. 2003/0137560 discloses a recording head using a cantilevered thermo-mechanical actuator including two layers, a heating layer and a dielectric material layer. An example of such a thermo-mechanical actuator will be briefly described with reference to FIGS. 5A to 5C.
FIG. 5A is a top plan view of a discharging portion of a recording head (the term “top plan view” refers to a view from the direction in which an ink droplet is discharged). FIG. 5B is a sectional view taken along line VB-VB of FIG. 5A. FIG. 5C illustrates a state in which a droplet is being discharged from the discharging portion shown in FIGS. 5A and 5B.
As shown in FIGS. 5A to 5B, a liquid chamber 2 is formed above a silicon substrate 1, and an ink droplet is discharged from a nozzle 3. A cantilever 4 serving as a thermo-mechanical actuator is formed in the liquid chamber 2. The cantilever 4 includes a heating layer 20, a conductor layer, and a dielectric material layer 21. The heating layer 20 is divided into two heating portions by a slit. The conductor layer forms an interconnect portion 5 (5a and 5b) that supplies current to the two heating portions, and an electrode 11 that connects the two heating portions. First, the heating layer 20 is formed. The conductor layer is superposed on the heating layer 20. Finally, the dielectric material layer 21 is superposed on the heating layer 20 and the conductive layer. In this way, the cantilever 4 is formed. The linear expansion coefficient of the dielectric material layer 21 is smaller than that of the heating layer 20. Being in contact with ink, the cantilever 4 is covered with a thin insulating film (not shown). When the two heating portions of the cantilever 4 are supplied with current and generate heat, the cantilever 4 bends upward (toward the nozzle 3) as shown in FIG. 5C due to the difference in linear expansion coefficient between the heating layer 20 and the dielectric material layer 21. Thereby, the ink 7 in the liquid chamber 2 is discharged from the nozzle 3 in the form of a droplet 8.
U.S. Pat. No. 6,598,960 discloses a cantilever 4 including a dielectric material layer 21 sandwiched between two heating layers 20, 20. In this example, first, the upper heating layer 20 is supplied with current, and thereby the cantilever 4 is caused to bend away from the nozzle 3. Next, the lower heating layer 20 is supplied with current, and thereby the cantilever 4 is caused to bend toward the nozzle 3 as shown in FIG. 5C. Thus, a droplet can be discharged by a large driving force.
U.S. Pat. Appl. Pub. No. 2004/0036739 discloses a trapezoidal cantilever 4 in which the width of the fixed end 9 is greater than the width of the free end 10. This can also generate a large driving force and suitably discharge a droplet 8.
In general, in a discharging portion of a recording head using a thermo-mechanical actuator, a cantilever serving as a thermo-mechanical actuator is repeatedly heated and cooled. Thereby, minute bubbles are generated. These bubbles gather together and accumulate in the form of large bubbles in the liquid chamber. In addition, since moisture in the ink evaporates due to heat through the nozzle, the ink is thickened. This hinders stable discharge. In order to prevent these bad effects, a pump (not shown) provided in the printer suctions ink through the nozzle and thereby removes the bubbles and refreshes the thickened ink.
However, in the above-described thermo-mechanical actuator, in order to sufficiently remove the bubbles and the thickened ink, it is necessary to exert a high suction pressure. If such a high suction pressure is exerted, as shown in FIG. 6, the free end 10 of the cantilever 4 serving as a thermo-mechanical actuator is displaced by the flow of the suctioned ink 7 and blocks the nozzle 3. In this state, if the suction is continued, the ink 7 cannot be suctioned. Therefore, the bubbles cannot be sufficiently removed. In addition, the thickened ink cannot be sufficiently refreshed.